The Lord of the Rings: Shadow of the East
Easterlings is a vague term for the wild tribes of eastern Middle-Earth. In the First Age the name is applied to the Peoples of Bor and Ulfang in Beleriand, the first ones who were faithful to the Eldar, the latter ones who were traitors and in secret allegiance with Morgoth became known as the “Wolf-folk”. The surviving Easterlings fled back in their eastern homelands after the drowning of Beleriand and became rulers of the indigenous tribes, most of whom were their distant kinsmen.
In the Second Age Khamûl was a fallen Easterling King who became a Nazgûl. In the Third Age the term Easterlings was used for the many tribes of Rhûn, maybe to some part relatives of the Easterlings of the first and second Ages. The original Easterlings of Rhûn were early enemies of Gondor who were defeated by Romendacil I in 500 TA and finally conquered by Romendacil II in 1248 TA.